I had a look at this linked site and came across their 18 August puzzle.
Rising to the bait of the "ferocious" status and not having used the site
previously I had a go.

I reached the following.
Can anyone suggest how to proceed?

524 973 186
197 486 532
638 -1- 749

941 7-3 2-8
872 4-1 9-3
365 829 417

413 697 825
786 -4- 391
259 138 674

I accept that I may have erred in reaching the above position and so
I give the start position as a challenge for someone else to derive the
full solution.

-24 --- ---
--7 -8- -3-
6-- -1- --9

--- 7-- 2-8
--- --- ---
3-5 --9 ---

4-- -9- --5
-8- -4- 3--
--- --- 67-

Only 19 initial values given - but should be soluble??

As a result of my experiences with this one, I have now declined to use
Sudoku San as a source of puzzles - preferring the "Daily" dose!
Can anyone restore my faith in the other site?
Where did I go wrong - before or after reaching 75 of the 81 cells?

I note, first, that the starting grid you exhibited contains 22 numbers, and not 19.

Here's what I've arrived at, after 12 moves. All of this agrees with the final, contradictory state you posted above. I'm pretty sure of all these moves, so you probably didn't make a mistake in the early going.

Code:

- 2 4 - 7 - - - -
- - 7 - 8 - - 3 -
6 - - - 1 - 7 - 9

- - - 7 - - 2 - 8
- 7 - - - - 9 - 3
3 - 5 8 - 9 - - 7

4 - - - 9 7 - - 5
7 8 - - 4 - 3 9 -
- - - - - 8 6 7 4

Just in case anyone else is following along, here's the sequence of moves. I think all of these are fairly obvious.

1. "3" at r5c9.
2. "4" at r9c9.
3. "9" at r8c8.
4. "9" at r5c7.
5. "7" at r1c5.
6. "7" at r3c7.
7. "7" at r6c9.
8. "7" at r5c2. This one wasn't so obvious. I had to locate the {2/8} pair in r5c1/r5c3 first.
9. "7" at r8c1.
10. "7" at r7c6.
11. "8" at r6c4.
12. "8" at r9c4.

If there are any more simple moves at this point, I'm not seeing them right now. Will let you know what else I figure out tomorrow sometime -- it''s time to go cook dinner. dcb

With Geoff's able assistance, I've now completed the puzzle. Looking at the grid that Alan posted to start this topic, it appears that his error occurred rather early -- around the 17th or 18th move, probably.

Here's the position from my previous post.

Code:

- 2 4 - 7 - - - -
- - 7 - 8 - - 3 -
6 - - - 1 - 7 - 9

- - - 7 - - 2 - 8
- 7 - - - - 9 - 3
3 - 5 8 - 9 - - 7

4 - - - 9 7 - - 5
7 8 - - 4 - 3 9 -
- - - - - 8 6 7 4

Here are the moves that Geoff pointed out. Keep in mind that we've already located a {2, 8} pair in row 5, at r5c1/r5c3.

13. "2" at r6c5 ("2"s already placed in rows 4 and 5, because of the pair)
14. "3" at r9c5 (because "5" must appear at either r9c1 or r9c2, and the only values missing in column 5 are {3, 5, 6})
15. "3" at r4c6 (because {5, 6} pair must now appear in r4c5/r5c5}
16. "3" at r1c4 (because it can't fit anywhere else in the first row)
17. "9" at r2c4 (it can't fit anywhere else in top middle 3x3 box)
18. "9" at r1c1 (because of "9"s in rows 2 & 3).

At this point the grid looks like this:

Code:

9 2 4 3 7 - - - -
- - 7 9 8 - - 3 -
6 - - - 1 - 7 - 9

- - - 7 - 3 2 - 8
- 7 - - - - 9 - 3
3 - 5 8 2 9 - - 7

4 - - - 9 7 - - 5
7 8 - - 4 - 3 9 -
- - - - 3 8 6 7 4

Differences between this position and the one Alan reached are fairly obvious. If I had to guess, I'd say you maybe wrote the "3" in r1c4 on the wrong side of the 7 at r1c5, Alan. Maybe not, but that's the sort of mistake I tend to make when I get in a hurry. dcb